This invention relates generally to magnetic imaging systems and specifically to a magnetic resonance imaging (MRI) magnet assembly.
There are various magnetic imaging systems which utilize permanent magnets. These systems include magnetic resonance imaging (MRI), magnetic resonance therapy (MRT) and nuclear magnetic resonance (NMR) systems. MRI systems are used to image a portion of a patient""s body. MRT systems are generally smaller and are used to monitor the placement of a surgical instrument inside the patient""s body. NMR systems are used to detect a signal from a material being imaged to determine the composition of the material.
These systems often utilize two or more permanent magnets directly attached to a support, frequently called a yoke. An imaging volume is providing between the magnets. A person or material is placed into an imaging volume and an image or signal is detected and then processed by a processor, such as a computer. The magnets are sometimes arranged in an assembly 1 of concentric rings of permanent magnet material, as shown in FIG. 1. For example, there may be two rings 3, 5 separated by a ring of non-magnetic material 7 in the gap between the magnet rings 3, 5. The ring of non-magnetic material 7 extends all the way through the magnet assembly 1 parallel to the direction of the magnetic field. The assembly 1 also contains a hole 9 adapted to receive a bolt which will fasten the assembly 1 to the yoke.
The prior art imaging systems also contains pole pieces and gradient coils adjacent to the imaging surface of the permanent magnets facing the imaging volume. The pole pieces are required to shape the magnetic field and to decrease or eliminate undesirable eddy currents which are created in the yoke and the imaging surface of the permanent magnets.
However, the pole pieces also interfere with the magnetic field generated by the permanent magnets. Thus, the pole pieces decrease the magnitude of the magnetic field generated by the permanent magnets that reaches the imaging volume. Thus, a larger amount of permanent magnets are required to generate a magnetic field of an acceptable strength in the imaging volume, especially in an MRI system, due to the presence of the pole pieces. The larger amount of the permanent magnets increases the cost of the magnets and increases the complexity of manufacture of the imaging systems, since the larger magnets are bulky and heavy.
Since the permanent magnets are strongly attracted to iron, the imaging systems, such as MRI systems, containing permanent magnets are assembled by a special robot or by sliding the permanent magnets along the portions of the yoke using a crank. If left unattached, the permanent magnets become flying missiles toward any iron object located nearby. Therefore, the standard manufacturing method of such imaging systems is complex and expensive because it requires a special robot and/or extreme precautions.
In accordance with one aspect of the present invention, there is provided an assembly for an imaging apparatus comprising at least one layer of soft magnetic material, and a body of a first material suitable for use as a permanent magnet having a first surface and a shaped second surface, wherein the first surface is attached over the at least one layer of the soft magnetic material and the second surface is adapted to face an imaging volume of the imaging apparatus.
In accordance with another aspect of the present invention, there is provided a magnetic imaging system, comprising a yoke comprising a first portion, a second portion and at least one third portion connecting the first and the second portions such that an imaging volume is formed between the first and the second portions, a first magnet assembly attached to the first yoke portion, wherein the first magnet assembly comprises at least one permanent magnet containing an imaging surface exposed to the imaging volume and at least one layer of a soft magnetic material between a back surface of the at least one permanent magnet and the first yoke portion, and a second magnet assembly attached to the second yoke portion, wherein the second magnet assembly comprises at least one permanent magnet containing an imaging surface exposed to the imaging volume and at least one layer of a soft magnetic material between a back surface of the at least one permanent magnet and the second yoke portion.
In accordance with another aspect of the present invention, there is provided an assembly suitable for use as a permanent magnet, comprising a base body suitable for use as a permanent magnet having a first and second major surfaces, and a hollow ring body suitable for use as a permanent magnet having a first and second major surfaces, where a first major surface of the hollow ring body is formed over a second major surface of the base body.
In accordance with another aspect of the present invention, there is provided a method of making an imaging device, comprising providing a support comprising a first portion, a second portion and at least one third portion connecting the first and the second portions such that an imaging volume is formed between the first and the second portions, attaching a first precursor body comprising a first unmagnetized material to the first support portion, attaching a second precursor body comprising a second unmagnetized material to the second support portion, magnetizing the first unmagnetized material to form a first permanent magnet body after the step of attaching the first precursor body, and magnetizing the second unmagnetized material to form a second permanent magnet body after the step of attaching the second precursor body.
In accordance with another aspect of the present invention, there is provided a method of making a magnet assembly, comprising placing a plurality of blocks of a material suitable for use as a permanent magnet into a mold cavity having a non-uniform cavity surface contour, filling the mold cavity with an adhesive substance to bind the plurality of blocks into a first assembly comprising a unitary body, such that a first surface of the unitary body forms a substantially inverse contour of the non-uniform mold cavity surface, and removing the first assembly from the mold cavity.
In accordance with another aspect of the present invention, there is provided a method of imaging a portion of a patient""s body using magnetic resonance imaging, comprising providing a magnetic image resonance system comprising a yoke comprising a first portion, a second portion and at least one third portion connecting the first and the second portions such that an imaging volume is formed between the first and the second portions, a first magnet assembly attached to the first yoke portion, wherein the first magnet assembly comprises at least one permanent magnet containing an imaging surface exposed to the imaging volume and at least one soft magnetic material layer between a back surface of the at least one permanent magnet and the first yoke portion, and a second magnet assembly attached to the second yoke portion, wherein the second magnet assembly comprises at least one permanent magnet containing an imaging surface exposed to the imaging volume and at least one soft magnetic material layer between a back surface of the at least one permanent magnet and the second yoke portion, detecting an image of a portion of a patient""s body located in the system, and processing the detected image.